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Assuming that the loading is even and the TC offsets are set equally, uneven temps in the kiln can be a simple programmable setting issue:

L&L Kilns with the DynaTrol 12-key controller made from 1/06 until 9/08(?) have their PID setting set to 100 rather than the factory setting of 65. This is not a really big deal but it can add to an existing problem.

When the kilns were fired before the PID setting existed, the center zone of the kiln only had to be on 30% to 40% of the time to keep up with the top zone and bottom zone- when the top and bottom zones were at 100%. In order to get the kiln to heat more quickly at the upper temps, the PID setting was added. It allows you to set the middle zone to come on at a set percentage- higher than 30%- 40% just to add more heat to the load when the bottom and top fire at 100%.

With the PID set to 100, the middle stays on 100% when the bottom and top stay on 100%. Because there is no top or bottom slab in the middle zone to suck away heat, the middle gets hotter and hotter and sometimes even results in an E-d/ Errd. As the elements age this setting will come on sooner and sooner because the elements are more and more tired and are working at 100% sooner and sooner. Loading heavily in the bottom or using shorter posts between shelves in the bottom will make the PID come on sooner as well. The sooner PID comes on, the more time it has to make the kiln fire more and more unevenly, until it is enough for the operator to notice.

Initially from 4/03 to about 4/04 or so the factory setting for PID was 85 (%). Around 4/04 it was lowered to 65 where it has stayed except for the controllers made between 1/06 and 9/08. 1/06 was the beginning of the 700 series boards and for some reason all these came with PID set to 100. Bartlett pre-sets a lot of settings in our DynaTrols and this is not checked when the kilns are made. (It is checked now until they start to come with the PID set to 65 again).

PID setting is easy to find. When the kiln says IDLE, press OTHER/ 4/ 4/ 3/ then press OTHER again and again until it reads PID. Press ENTER and see the setting. Ones made after 1/06 will just read a number like 100 or 65. Ones made before 1/06 will read a number flashing back and forth with 'PCt' meaning 'percent'. With either, set it to 65- or as low as 40 if they still complain the center is too hot.

Additional Actions to Take

Why Calibrate Your Kiln?

Most people will be satisfied with the operation of their kiln right out of the box.

However for the most precise or accurate work a kiln needs to be calibrated or adjusted to match the load, the temperature that you typically fire to, the unique characteristics of the kiln, and the thermocouples.

It is important to establish a relationship between you, your kiln and your work.

Although we use the word “calibrate” or “tuning” really what is called for is an understanding of the heating characteristics of your particular kiln, and understanding of how thermocouples and controls work and their limitations, how you load your kiln, the clay and glazes that you use, how pyrometric cones work, and the basic concepts of heat-work.

The heating characteristics of your kiln

Each kiln is different. They vary from brand to brand, kiln series to kiln series, kiln size to kiln size, and even for identical models from kiln to kiln.

The age of the elements, the insulation thickness, the distance from the center of the kiln to the elements, the kiln height, how tight the kiln is, if there is a vent, the condition of the thermocouples as they age, whether there is a protection tube on the thermocouple, etc. all make a difference.

Even external conditions like voltage and temperature in the room affect the firing.

The controls used in most kilns, while perfectly acceptable for ceramics, are not military precision instruments.

A kiln is not a standardized “instrument” – it is more like a violin that needs to be tuned and understood.

Controls and Thermocouples

The controls in ceramic kilns are typically accurate to about 1% of scale vs. something like .1% of scale in a precision industrial control.

The thermocouples can read +/- 10°F from actual temperature even when when they are brand new. (This is a function of the variation in materials in the thermocouple. Even though L&L uses "Special Limit of Error" wire, there are still all kinds of variation in the thermocouples, the thermocouple circuit, and the cold compensation on the control. See this on Wikipedia for more information on thermocouples.

In addition to this Type K thermocouples, the most common ones used in ceramic kilns, will drift over time, especially when used over 2000°F.

Type S platinum thermocouples do not drift at those temperatures. Although they are available as options on most L&L kilns and most other brands, they cost on the order of $230 each.

The thermocouple protection tube that L&L uses introduces a known 18°F offset into the thermocouple reading. Although it is a known constant and is adjusted for, the exact amount can vary slightly based on individual tubes (this does not change with time).

The temperature that you see represented on the control will probably not be completely accurate (and even if it were it is only one piece of the information you need to understand what is happening in the kiln).

On the other hand it tends to be consistent over time (except for the thermocouple drift). Hence, when you “calibrate” the control to meet the requirements of your work, the experience of many thousands of customers tells us that you will get consistent results firing to firing, all other things being equal.

How you load your kiln

How you load your kiln makes a huge difference in how heat reaches and affects your work.

For example, fire an empty kiln and you will probably get error codes.

Fire it too densely or too heavily in the top or bottom and you might get different error codes.

Ideally you will load it with a reasonable amount of work, not too close the elements, and distribute the work as evenly as possible.

Don’t put a shelf right next to an element. Most of the heating about 1200°F is radiant heat. Because not all of the work is exposed directly to the elements you need to make space for the convective heat to circulate. Also you need to allow time for the heat to get from the wall elements to the inside of the kiln while not overheating the work towards the outside.

Clay and Glazes

Talk to your clay and glaze suppliers to find out their firing recommendations.

Some glazes can be very fussy about heat-work, others not so much.

The main point, however, is to “tune” or “calibrate” the kiln to the appropriate level of precision that you need. Don’t overdo it.

Also be aware that clay and glaze vary with different batches.

Heat-Work

Ceramics and glazes develop from heat-work, which is a function of time and temperature.

So, for any given temperature, the amount of time spent at that temperature would determine how much heat-work is done.

Looking at one part of this equation can be misleading.

Assuming you had a perfectly known temperature you would then have to determine how much time at that temperature will give you the right result.

Cones

Pyrometric Cones are probably the best way to “see” ceramic heat-work. They represent and reflect the actual ceramic process taking place. They are far more representative of what you want to see than temperatures as shown on the control.

Even pyrometric cones are telling a story that is only representative of what your work will experience.

Where they are placed in the kiln can make a difference, for instance.

They will always need to be interpreted based on your experience with your own work.

Calibrating the kiln with cones is your best chance of coming close to what you want.

Remember: The kiln calibration only has to be as good as what you need to produce the work you want – not what the cones say.

Repeatable Results

Given all the variables involved the only way to get completely consistent results is to repeat the same process, with the same time-temperature cycle, with a kiln that changes as little as possible, and with the same materials as possible.

This is how industrial ceramics is done but with far higher precision and with process engineers using statistical process control.

Most art ceramics has a much higher degree of variability.

There are exceptions to this rule and those who pursue those exceptions need to and often do put a tremendous amount of time and effort into this achievement. Some of the crystalline ceramic artists out there are good examples of this.

How to Calibrate Your Kiln

Tools for Calibrating your Kiln

There are two basic tools to calibrating your kiln: thermocouple offset and cone offset.

Pyrometric cones are used as the main tool to "see" what is happening in the kiln.

FIRING A KILN WITH WITNESS CONES

SOMETIMES AN EMPTY KILN WILL NOT GET TO TEMPERATURE

Sometimes a new kiln does not get to temperature during the test firing.

This is generally because of an empty kiln.

It is generally a good idea to fire with your kiln furniture to put some mass in the kiln even during the test firing.

DIFFERENCES IN AN EMPTY KILN VS. FULL KILN

One difference between an empty and full kiln is that an empty kiln cools a lot quicker which will freeze the cone very quickly. In a full kiln there is a lot of mass in the kiln that is just as hot as the kiln around it. It is this mass (the load in the kiln), which is radiating it’s heat as well, that will continue to melt the cone for a little longer after the kiln has been shut down.

Once the kiln is fine-tuned, it is the variable of how you have loaded the kiln that will account for many of the variations you will see from firing to firing.

Loading will affect the speed of firing - an empty kiln will fire differently than a full one. Although the control does compensate for this that compensation is not totally perfect.

ADJUSTING / CALIBRATING THE KILN WITH THE THERMOCOUPLE OFFSET

The kiln cannot be calibrated until it has reached temperature and affected a witness cone. The reaction of the witness cone to the firing is how you begin the calibration process. (Kilns are not fired before they ship).

You can fine-tune how the kiln reads temperature by adjusting the Thermocouple Offset. (See below for how to do this).

Fire the kiln with witness cones. Use the most typical or critical cone you fire to. Typically the most important cone you will fire to is your glaze temperature. Generally, bisque temperatures are less critical.

Thermocouple offset will change the reading of the thermocouples the same amount from room temperature to the maximum temperature of the kiln. (Cone Offset, which will be covered later on, will adjust how the kiln reacts to a particular cone/temperature).

WHAT IS THERMOCOUPLE OFFSET?

Adding thermocouple offset lowers the temperature in the kiln (relative to the temperature reading on the control. For instance lets say the control reads 2000°F and you add 10°F of thermocouple offset. Now the control thinks there is 10°F more in the kiln and, for a setpoint of 2000°F, it will actually control to 1990°F in the kiln.

Subtracting thermocouple offset raises the temperature in the kiln. For instance lets say the control reads 2000°F and you subtract 10°F of thermocouple offset. Now the control thinks there is 10°F less in the kiln and, for a setpoint of 2000°F, it will actually control to 2010°F in the kiln.

Thermocouple Offset affects both Easy-Fire programs and Vary-Fire programs on a Dyna-Trol or Bisque & Glaze Programs and Custom Programs on a One-Touch (Cone Offset, on the other hand, only affects Easy-Fire programs and only on a Dyna-Trol).

All temperature readings that you see in your control are interpreted by the control from the thermocouple signal. Thermocouple readings can be inaccurate from thermocouple drift or signal change through many factors like inductive current. The only absolute that you can have good confidence in is a witness cone. When we change the thermocouple offset we are not changing the setpoint of the control only the temperature interpretation with thermocouple offset.

You can think of the control setpoint as the other absolute and the thermocouple signal reading as the one variable that gets changed.

When we add temperature to the thermocouple signal we make the control believe the kiln is hotter than it would otherwise think and it will shut off sooner and fire cooler.

When we subtract temperature from the thermocouple signal we make the control believe the kiln is cooler than it would otherwise think and it will shut off later and fire hotter.

EXAMPLE AND PROCESS

If the witness cone bent slightly during the first firing, but no more than a little bit, then start by reducing the thermocouple offset setting by 5°F to make the kiln fire slightly hotter.

If the witness cone did not bend at all, then you can start by reducing the thermocouple offset setting by 10°F to make the kiln fire hotter.

If the witness cone bent a little too much, you might wait and see how it does with a full load, or you could start by increasing the thermocouple offset by 5°F to make the kiln fire a little cooler.

If the witness cone bent more than a little bit start by reducing the thermocouple offset settings 10°F which will make the kiln fire cooler.

If the witness cone collapsed start by reducing the thermocouple offset settings 15°F.

There are beginning suggestions - feel free to experiment outside of this.

ANOTHER METHOD

To determine an good starting point for thermocouple offset (or cone offset) you can fire the kiln a cone or so hotter than your witness cone.

Then carefully observe when the cone bends and note the temperature displayed on the control at that moment.

Compare this number with what it you think it ought to be using an Orton Cone Chart. You will have to have some idea of the speed of firing to get an accurate number.

It will flash between °FOS (which stands for Deg F Offset) and 0018 (The 0018 stands for a thermocouple offset of 18°F - which comes preprogrammed into the control to compensate for the ceramic protection tubes. By changing the offset to 0010 we are REDUCING the offset by 8°F- making it fire 8 deg hotter). (NOTE: On older kilns with a slightly different composition thermocouple protection tube the preprogrammed value is 0050).

Press 0008 to reduce thermocouple offset by 10°F.

Press 0013 to reduce thermocouple offset by 5°F.

Press ENTER to accept your input.

Do the same for all your thermocouples. The prompts will scroll past in the order of TC1, TC2 and TC3.

VIDEO SHOWING HOW TO CHANGE THERMOCOUPLE OFFSET ON A DYNATROL

HOW TO CHANGE THERMOCOUPLE OFFSET IN A ONE-TOUCH CONTROL: STEP BY STEP

Access the Thermocouple Offset by getting into the Options.

Options are accessed by holding the ENTER button while turning the power onto the control (turn on the kiln with the toggle switch) and continuing to hold onto the ENTER button until EdIt is displayed.

This activates the Options Menu.

The first thing you will see after turning the power on while pressing ENTER is LL-G or 1t-1 (This is the software version).

Then you will see 1288 if it is a Cone 10 control or 1249 if it is a Cone 6 control.

Then you will see EdIt and you will hear a beep. You can now let go of the ENTER button.

Press ENTER again. You will see °F or °C.

Press ENTER again and you will see OFFS

Press the UP arrow to enter a positive offset OR press the DOWN arrow to add a negative sign to the offset. All this first button press does is get you in the right range.

You can NOW use the UP or DOWN arrow to change the value of the offset.

When you are in the positive range you can only go as low as 0000. The UP arrow increases the offset setting and the DOWN arrow decreases it.

When you are in the negative range you can only go as high as -000. The UP arrow, when you are in the negative range, is actually moving the setting in the negative range. For instance if you start at -000 and hit the UP arrow you will be at -001 (which is negative by one degree). If you then hit the DOWN arrow you will go back to -000 (which is actually in the "postive" direction). Some of this is counterintuitive so it make take some getting used to by just experiementing.

Once you are done with your setting hit ENTER and in a few seonds the control will flash between IDLE and some temperature and STOP.

If you want to redo the offset setting you have to turn the control off and start over.

The control comes with a pre-programmed +18 Deg F offset to compensate for the thermocouple protection tube.

Note that the control does not convert the offset setting to deg C or F. If you are operating in Deg C then the offset setting should be +8 Deg C.

Note: if you first press the DOWN button you can only set a negative value or if you first press the UP button you can only enter a positive value.

You can go back and change this later if you make a mistake.

CONE OFFSET ON A ONE-TOUCH CONTROL

There is no Cone Offset Option on the One-Touch Control

ADJUSTING FOR TEMPERATURE VARIATION TOP TO BOTTOM IN THE KILN IN A ZONE CONTROLLED KILN

Note that you can use different Thermocouple Offsets for the three (or two) different thermocouples.

This allows you to make the top or bottom hotter or cooler to even out the temperatures in a kiln.

CONE OFFSETS ON A DYNATROL

CNOS (Cone Offset) - is used to fine tune what the Dynatrol thinks the final cone temperature should be in EASY-FIRE programs. The final cone temperature can be raised or lowered a maximum of 99°F (or 55°C). When entering the offset temperature the following code is used: the left two digits designate whether to raise (00) or lower (90) the cone temperature, that is, “00” means plus (+) and “90” means minus (-). The right two digits are the number of degrees the cone temperature will be raised or lowered. This offset will remain programmed only for the specific cone number until you reprogram the cone offset differently

Examples:

Number

Meaning

0020

Raise the final cone temperature by 20°F

0040

Raise the final cone temperature by 40°F

0015

Raise the final cone temperature by 15°F

9030

Lower the final cone temperature by 30°F

9005

Lower the final cone temperature by 5°F

9045

Lower the final cone temperature by 45°F

NOTE: This option does not affect the VARY-FIRE (Ramp-Hold) mode but it will show up on the menu.

NOTE ABOUT PREPROGRAMMED CONE OFFSETS:

The Cone Offsets come preprogramed. From cone 022 to cone 017 the cone offsets are set at 9020. All other cones are preset at 0000.

Note on Blue DynaTrols made before Oct 1 2004 the cone offset was 9030 for cones 022 to 017 and 9020 for other cones. (The offsets were changed when we switched to a more responsive thermocouple protection tube).

You can always change offsets.

The RESET option in Other menu will NOT reset these cone offset settings.

This is part of the compensation necessary for the mullite thermocouple protection tubes.